The present invention generally relates to an ion source. More particularly, the invention concerns an ion source which can be advantageously employed in fine processing of semiconductors, thin films or the like by making use of an ion beam.
In recent years, in accompaniment to tendency of dry semiconductor processes being increasingly adopted, there arises a great demand for an etching apparatus in which an ion beam is made use of. Implementation of semiconductor devices with high fineness has made a remarkable progress, as demonstrated by the fact that a minimum line width of the submicron order is being realized in the fabrication of LSI devices. In light of the present state of the art, the dry etching apparatus destined to perform the process with high fineness of the submicron order is imposed in respect to the performance capability thereof with such requirements that the divergence angle of the ion beam is constricted to a very small value for satisfying the desired high processing precision and that the ion beam should have a very uniform density. As exemplary structure of the ion beam etching apparatus of this sort, there may be mentioned the structure disclosed in "Extended Abstracts for The 32nd Spring Meeting, Mar. 29-Apr. 1, 1985 of The Japan Society of Applied Physics and Related Societies", p. 316. The ion beam etching apparatus disclosed in the above publication is composed of a cylindrical vacuum container serving as a vacuum chamber, a microwave waveguide connected to one end of the vacuum container, and a coil disposed around the outer periphery of the vacuum container for generating a magnetic field of the direction which is in parallel with the direction the microwave energy is introduced. A microwave power is supplied from a microwave oscillator (not shown). When the angular frequency of the microwave is represented by .omega. dimensions are so selected that the following relation can be satisfied at least approximately: ##EQU1## where m.sub.e represents mass of electron, e represents electric charge of electron, and B represents density of magnetic flux. When the microwave of 2.45 MHz is used, the density of magnetic flux (B) is set approximately equal to 875 Gauss.
When the above conditions are met, electrons are accelerated due to cyclotron resonance of electrons taking place within the vacuum chamber, resulting in that ionization is promoted intensively by the accelerated electrons, whereby a plasma is produced. From this plasma, ions are extacted through ion extracting means including an electrode biased with a positive voltage, an electrode biased with a negative voltage and an electrode applied with the ground potential. This known ion source suffers however a shortcoming that the uniformity of the plasma density distribution is poor with an axial or center portion of the plasma exhibiting a higher density. As a consequence, the density of the extracted ion beam is high only at the axial center portion thereof. In other words, the ion beam as extracted is of nonuniformity in the ion density distribution in the radial direction thereof. Further, since the magnetic lines of force generated by the coil tend to leak toward the ion extracting electrode array, the paths of ions tend to be bent, giving rise to a problem that the divergence angle of the ion beam is increased.